One would expect that an improvement in exercise performance following improvement in central hemodynamics with cardiac resynchronization therapy (CRT) would be associated with improved muscular blood flow and energy metabolism. However, so far no reports have been published on the skeletal muscle response to CRT. The purpose of this study was to evaluate the effect of 6 months CRT pacing on skeletal muscle histology and mitochondrial mass and the association of these changes to alterations in functional capacity as measured with peak VO2. Moreover, we also sought to assess the relationship between changes in skeletal muscle and alterations in the inflammatory response in serum and in skeletal muscle.